You must have a custom Work Environment loaded.
Try the 'Standard' WE.
This was a new command in 18, so if you have a WE from a previous version you will not see this command unless you add it yourself or load the 'Standard' WE.
You can also "Save As" in the 3D window and change the file type to BIMx (not possible in versions after 18).
One of the forum moderators. Versions 6.5 to 27 Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10 Lenovo Thinkpad - i7-1270P 2.20 GHz, 32GB RAM, Nvidia T550, Windows 11
ok. Have reverted the work environment and it has reappeared. Not sure why it went in the first place as I had not changed my work environment (customised & saved) in some time, so it had disappeared from my customised environment.