I've been using an iphone in lockdown mode for a while now, simply to disable a number of the poor security design choices Apple made.

One thing I just noticed this last week after upgrading to 18.x is that lockdown mode also disables RCS. I found I couldn't enable RCS, so turned lockdown mode off, found RCS now worked, then turned lockdown back on.

I can live without RCS mode.

For me, the one real annoyance of lockdown mode is that it disables 2G connectivity. Oh well, I guess one can carry a second 2G capable feature phone.