We use Sharepoint 2016 and 2019.
Both are patched by WSUS. Starting in December '19, the updates for these products where no longer downloaded by WSUS, nor can they be imported manually.
Can you please confirm this?
In WSUS (version does not matter), right click the updates section and search for update ID 4484215 (january 2020 update for sharepoint 2016) - does your WSUS find it?
To be detected, the office 2016 product needs to be enabled, of course (Sharepoint is part of office in WSUS)
For those of you, who have office 2019 enabled in WSUS, please try to search for update ID 4484224
I suspect that MS has screwed up something, since it stopped to detect these updates in december already.