Aye, I think any MSI MAX boards will be fine as that was kind of the point they were released in the first place. I think my X470 Taichi Ultimate will probably get Zen 3 support, but likely very late and possibly only one early AGESA. My PRIME X370-PRO is still stuck on AGESA 1.0.0.3ABBA so I doubt that'll get any more BIOS updates.The tomahawk max page states it will be covered for future CPU upgrades.
https://www.msi.com/blog/msis-max-motherboard-lineup
taken from this page.
e) You want a value-oriented motherboard that’ll support not only the latest AMD releases but will also have you covered for all future AM4 product releases.
Infact this covers all MSI max boards stating support for future AM4 CPU.
Not making it mandatory for at least B450 boards to support Zen 3 is a dick move though. Not as bad as what Intel does with chipsets, but still bad.