It depends on the manager and the service.

The issue with decrypting a private key online is that it'll be in the most vulnerable place for an attack - the RAM and the CACHE.

There are some open coufce password managers you can get thst work on your computer and store the encrypted information only on your computer too which are fairly secure but ultimately not perfect.

If you're referring to a hardware wallet like a ledger or a trezor instead then they're considered one of the most secure ways of storing your keys at the moment.