MongoDB hosting options now that Heroku mLab add-on is being removed

mLab was acquired by MongoDB, its users are encouraged and will eventually need to migrate to MongoDB Atlas.

There is no add-on required to use Atlas, and Atlas provides a free tier. I am unfamiliar with mLab costs but I imagine depending on usage Atlas may be cheaper or more expensive than mLab.

You can use other hosted MongoDB services as the other answer mentions. Due to MongoDB license change to SSPL the other services are now required to negotiate individual agreements with MongoDB Inc. to offer MongoDB database as a service. Some services (e.g. ObjectRocket) have done this and are offering current MongoDB versions (4.4 as of this writing); others haven't and are limited to 4.0 and earlier versions.

mLab isn't the only MongoDB game in town: the ObjectRocket for MongoDB addon looks like another easy option.

Alternatively, if you prefer, you could continue to use mLab, but not via an addon. You'd just have to point your application at your separately managed mLab database.

Pretty straightforward comment above answered - "You do not need an addon to use Atlas. – D. SM 17 hours ago" So just go ahead with the Sandbox free tier.