Certainly 100% legal to identify with CW on VHF / UHF FM repeaters; however, the issue it brings is courtesy to others / social norms depending on the situation. Legality does not equate morality, nor does it define what should be deemed socially appropriate... Good judgment & principle in social situations is not something that can be legally defined, nor cleared up by increasing granularity of how laws are to be enforced. Those who require this type of direct government babysitting & oversight should probably seek professional therapy from a clinical psychologist for an undiagnosed personality disorder... There is the issue of private property & rules imposed by the repeater owner: If they ask you to behave & communicate in certain ways, while not illegal for you to disobey in a legal manner by passive aggressively / incompetently using CW, they have EVERY right to ban you from using their private property, where if you continue on, then you're now operating illegally...
For an extreme case: On a 2m or 70cm net on a repeater, while legal, would it be appropriate to check in, answer the dumb question & make some long winded random comment about what you had for dinner last night, & close communication all in CW? No, & not one person would think so, and everyone would deem you a pretentious jerk for disrupting the net... However, it would be perfectly legal, until you're asked to no longer participate, where you will be illegally disrupting legal communications in a willful & intentional manner if you do continue on...
When casually talking with a group of friends on a linked repeater system "after hours" that spreads over a wide area, would you think it appropriate for a breaking station to jump in to key their call in CW, rather than quickly state "break" & wait to speak their call during a given pause in communication? It would be perfectly legal for them to do so, but I'd fathom you'd not want to respond to, nor want to converse with someone who's so obtuse & disrespectful in their communication "style" compared to the socially accepted norm.
When casually talking with a group of friends on a local repeater late at night, perfectly legal & possibly socially acceptable to simply hit the button to CW your call to remain compliant with the law depending on your area & the specific situation (so long as it's permitted by the private owner of the repeater system); however, you find this more on HF bands where CW is much more understood language by random folks who may be listening. It's really dependent upon the specific situation whether it's "OK" in a social aspect to CW your call to remain legally compliant. This really shouldn't have to be laid out in black & white... The thing is, Technicians are frequently not versed in CW, and many of the folks who may be listening won't be able to identify your CW call.. While legal to do do so, & possibly socially acceptable as well, CW calling during a voice FM conversation is kind of a pretentious act on VHF & UHF due to this being where Technician licensed operators primarily work.
If you're at a bar with a bunch of your friends, where you're all speaking English by voice & don't know Spanish, would you deem it appropriate if someone jumped into the conversation speaking Spanish, even though they spoke perfectly good English as their native language? No you wouldn't; however, they would be acting perfectly legal... Would you find this character a bit odd, off putting, & incredibly annoying if this random fellow introduced himself in Spanish that you don't understand, spoke perfect English throughout a brief conversation, then exited the conversation by speaking in Spanish that you don't understand? Perfectly legal if he did, but it's also really stupid that he did & incredibly annoying to anyone else who's listening & doesn't know Spanish... On VHF & UHF especially, CW isn't a common language for information exchange, where it's a bit prickish to speak in languages that your audience does not understand...
The legality aspect, simply requires you to identify your transmissions with your call sign in a manner that is identifiable in a prescribed time frame. The legality aspect does not, & should not define what is deemed socially acceptable between consenting parties, where it depends on the specific situation whether it is OK or not to CW your call to remain legally compliant in documenting / owning your transmissions. Requiring that government define what is deemed socially acceptable or not in various situations & circumstances, is a sign that you likely require professional cognitive therapy help that government simply cannot provide you...