It is working in “sandbox” environment since mid of this year. We are still ironing out some issues. You can see updates here: Migration from 3.x to 5.x (#344) · Issues · Turris / Turris OS / Turris OS packages · GitLab
The main blocker before we release it for public testing are some hacks in updater (lists/migrate3x: Add provides and virtuals to pass migration (!59) · Merge requests · Turris / Turris OS / Turris Build · GitLab).
Secondary, automatic migration (meaning not triggered by user) is going to come with Turris 5.x rather than 4.x because of missing samba4 in 4.x.
It is recommended. Just know that it won’t be without issues (but migration won’t be either). I think that currently some stuff are even more stable on 4.x+ over 3.x. Don’t get confused by reports of issues to something that is not yet part of core system (reForis is only experimental). Note that some features are and will be missing. Data collection is not yet fully done but dynamic firewall is present. Ssbackup and mobile app are no longer supported with 4…x. Kernel changed a lot and some hardware can (including on board switch chip) are supported differently. On the other hand regarding stability it is an improvement I would say.