Where are all past Salesforce Release Notes
Spring '21 (v51.0) https://resources.docs.salesforce.com/230/latest/en-us/sfdc/pdf/salesforce_spring21_release_notes.pdf
Winter '21 (v50.0) https://resources.docs.salesforce.com/228/latest/en-us/sfdc/pdf/salesforce_winter21_release_notes.pdf
Summer '20 (v49.0) https://resources.docs.salesforce.com/226/latest/en-us/sfdc/pdf/salesforce_summer20_release_notes.pdf
Spring '20 (v48.0) https://resources.docs.salesforce.com/224/latest/en-us/sfdc/pdf/salesforce_spring20_release_notes.pdf
Winter '20 (v47.0) https://resources.docs.salesforce.com/222/latest/en-us/sfdc/pdf/salesforce_winter20_release_notes.pdf
Summer '19 (v46.0) https://resources.docs.salesforce.com/220/latest/en-us/sfdc/pdf/salesforce_summer19_release_notes.pdf
- Spring '19 (v45.0) https://resources.docs.salesforce.com/218/latest/en-us/sfdc/pdf/salesforce_spring19_release_notes.pdf
- Winter '19 (v44.0) https://resources.docs.salesforce.com/216/latest/en-us/sfdc/pdf/salesforce_winter19_release_notes.pdf
- Summer '18 (v43.0) https://resources.docs.salesforce.com/214/latest/en-us/sfdc/pdf/salesforce_summer18_release_notes.pdf
- Spring '18 (v42.0) https://resources.docs.salesforce.com/212/latest/en-us/sfdc/pdf/salesforce_spring18_release_notes.pdf
Winter '18 (v41.0) http://resources.docs.salesforce.com/210/latest/en-us/sfdc/pdf/salesforce_winter18_release_notes.pdf
Summer '17 (v40.0) http://resources.docs.salesforce.com/208/latest/en-us/sfdc/pdf/salesforce_summer17_release_notes.pdf
Spring '17 (v39.0) http://resources.docs.salesforce.com/206/latest/en-us/sfdc/pdf/salesforce_spring17_release_notes.pdf
Winter '17 (v38.0) http://resources.docs.salesforce.com/204/latest/en-us/sfdc/pdf/salesforce_winter17_release_notes.pdf
Summer '16 (v37.0) http://resources.docs.salesforce.com/202/latest/en-us/sfdc/pdf/salesforce_summer16_release_notes.pdf
Spring '16 (v36.0) http://resources.docs.salesforce.com/200/latest/en-us/sfdc/pdf/salesforce_spring16_release_notes.pdf
Winter '16 (v35.0) http://resources.docs.salesforce.com/198/latest/en-us/sfdc/pdf/salesforce_winter16_release_notes.pdf
- Summer '15 (v34.0) http://resources.docs.salesforce.com/196/latest/en-us/sfdc/pdf/salesforce_summer15_release_notes.pdf
- Spring '15 (v33.0) http://resources.docs.salesforce.com/194/latest/en-us/sfdc/pdf/salesforce_spring15_release_notes.pdf
- Winter '15 (v32.0) http://resources.docs.salesforce.com/192/latest/en-us/sfdc/pdf/salesforce_winter15_release_notes.pdf
- Summer '14 (v31.0) http://resources.docs.salesforce.com/190/latest/en-us/sfdc/pdf/salesforce_summer14_release_notes.pdf
- Spring '14 (v30.0) http://resources.docs.salesforce.com/188/latest/en-us/sfdc/pdf/salesforce_spring14_release_notes.pdf
- Winter '14 (v29.0) http://resources.docs.salesforce.com/186/latest/en-us/sfdc/pdf/salesforce_winter14_release_notes.pdf
- Summer '13 (v28.0) http://resources.docs.salesforce.com/184/latest/en-us/sfdc/pdf/salesforce_summer13_release_notes.pdf
- Spring '13 (v27.0) http://resources.docs.salesforce.com/182/latest/en-us/sfdc/pdf/salesforce_spring13_release_notes.pdf
- Winter '13 (v26.0) http://resources.docs.salesforce.com/180/latest/en-us/sfdc/pdf/salesforce_winter13_release_notes.pdf
- Summer '12 (v25.0) http://resources.docs.salesforce.com/178/latest/en-us/sfdc/pdf/salesforce_summer12_release_notes.pdf
- Spring '12 (v24.0) http://resources.docs.salesforce.com/176/latest/en-us/sfdc/pdf/salesforce_spring12_release_notes.pdf
- Winter '12 (v23.0) http://resources.docs.salesforce.com/174/latest/en-us/sfdc/pdf/salesforce_winter12_release_notes.pdf
- Summer '11 (v22.0) http://resources.docs.salesforce.com/172/latest/en-us/sfdc/pdf/salesforce_summer11_release_notes.pdf
- Spring '11 (v21.0) http://resources.docs.salesforce.com/170/latest/en-us/sfdc/pdf/salesforce_spring11_release_notes.pdf
- Winter '11 (v20.0) http://resources.docs.salesforce.com/168/latest/en-us/sfdc/pdf/salesforce_winter11_release_notes.pdf
- Summer '10 (v19.0) http://resources.docs.salesforce.com/166/latest/en-us/sfdc/pdf/salesforce_summer10_release_notes.pdf
- Spring '10 (v18.0) http://resources.docs.salesforce.com/164/latest/en-us/sfdc/pdf/salesforce_spring10_release_notes.pdf
- Winter '10 (v17.0) http://resources.docs.salesforce.com/162/latest/en-us/sfdc/pdf/salesforce_winter10_release_notes.pdf
- Summer '09 (v16.0) http://resources.docs.salesforce.com/160/latest/en-us/sfdc/pdf/salesforce_summer09_release_notes.pdf
- Spring '09 (v15.0) http://resources.docs.salesforce.com/158/latest/en-us/sfdc/pdf/salesforce_spring09_release_notes.pdf
- Winter '09 (v14.0) http://resources.docs.salesforce.com/156/latest/en-us/sfdc/pdf/salesforce_winter09_release_notes.pdf
- Summer '08 (v13.0) http://resources.docs.salesforce.com/154/latest/en-us/sfdc/pdf/salesforce_summer08_release_notes.pdf
- Spring '08 (v12.0) http://resources.docs.salesforce.com/152/latest/en-us/sfdc/pdf/salesforce_spring08_release_notes.pdf
- Winter '08 (v11.0) http://resources.docs.salesforce.com/150/latest/en-us/sfdc/pdf/salesforce_winter08_release_notes.pdf
- Summer '07 (v10.0) http://resources.docs.salesforce.com/148/latest/en-us/sfdc/pdf/salesforce_summer07_release_notes.pdf
- Spring '07 (v9.0) http://resources.docs.salesforce.com/146/latest/en-us/sfdc/pdf/salesforce_spring07_release_notes.pdf
- Winter '07 (v8.0) http://resources.docs.salesforce.com/144/latest/en-us/sfdc/pdf/salesforce_winter07_release_notes.pdf
- Summer '06 (v7.0) http://resources.docs.salesforce.com/142/latest/en-us/sfdc/pdf/salesforce_summer06_release_notes.pdf
- Spring '06 (v6.0) Not released
- Winter '06 (v5.0) http://resources.docs.salesforce.com/142/latest/en-us/sfdc/pdf/salesforce_winter06_release_notes.pdf
- Summer '05 (v4.0) http://resources.docs.salesforce.com/142/latest/en-us/sfdc/pdf/salesforce_summer05_release_notes.pdf
- Spring '05 (v3.0) Not released
- Winter '05 (v2.0) http://resources.docs.salesforce.com/142/latest/en-us/sfdc/pdf/salesforce_winter05_release_notes.pdf
- Summer '04 (v1.0) http://resources.docs.salesforce.com/142/latest/en-us/sfdc/pdf/salesforce_summer04_release_notes.pdf
- Spring '04 (v?.?) http://resources.docs.salesforce.com/142/latest/en-us/sfdc/pdf/salesforce_spring04_release_notes.pdf
- Winter '04 (v?.?) http://resources.docs.salesforce.com/142/latest/en-us/sfdc/pdf/salesforce_winter04_release_notes.pdf
Source: https://help.salesforce.com/apex/HTViewHelpDoc?id=whats_new.htm&language=en_US
Not Released
It turns out that Spring '05 and Spring '06 were not released. Hence--no release notes!
- https://twitter.com/salesforcedocs/status/695308101667414016
Matching Releases to API versions
You can use /services/data/
to get XML that links from the release name to the API version from Winter '11 onward. [Source]
You can always find the list here: https://help.salesforce.com/apex/HTViewHelpDoc?id=whats_new.htm&language=en_US
Should the link change in future searching for "past releases" should find it.
The above answer has all the links and it should be upvoted but isn't it easy to go to developer.force.com and search for release notes.
You can check using the below link: https://developer.salesforce.com/search?q=release%20notes&language=en