Can I check playtime on PS4 without PS Plus?

No There Is no official way for a user to check their playtime statistics. And as for checking RDR2 playtime exclusively Rockstar didnt include the ability to check it unlike their former games (you could check a lot of your in-game stats on their website for GTA-V etc. ). You can enable an email from playstation that will send you your total playtime for the year but you can't check this whenever you want.

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Yes, you can check all stats including playtime on rockstar's social club website. Just login with your platform account.