Probably.
The foundation of /e/OS, however you might perceive it, doesn’t say that Android versions and phones can’t fall out of support. Murena are operating within existing technical constraints and certainly not with a limitless budget, i.e. they are operating in reality and can’t do magic … see also Why has support for a large number of phones been stopped?
Click/tap on the avatar circle or username of the user you want to message, the resulting popup will feature a “Message” button.
If the button is missing for you, the forum probably still sees you as a new user and applies some restrictions … Understanding Discourse Trust Levels