Now that disablelocal is gone, how to stop local (internal drive) TimeMachine backups? For years now everyone who needs backup just leaves a now-cheap ultra fast external drive continually plugged in to the back of their mac or macbook, job done.

To disable the "local" (internal drive) backups, you would just

$ tmutil disablelocal


This is gone in 10.15.2  :/

How to completely stop local time machine backups on 10.15.2 ?
Keeping an external drive connected cleans up snapshots in 24 hours. There’s no real need to disable things in my experience, other than let the hourly backups run and prune.
I would also upgrade to 10.15.7 and then report if you have some specific issue once a day passes on the new system build.
With APFS on SSD I can’t measure any performance issue since copy on write is basically handled at full speed via non-blocking IO.
The old mechanism absolutely had a performance impact so strategic cases, disabling made an effect. If you are on a hard disk and APFS, you have much bigger performance problems in general with APFS than managing the snapshots.