That isn’t good solution at all. And most of CTAs are just for annoy mobile using people enough that they either registrate or leave.
All of users know really good they can registrate. They do that if they feel there is some real reasons or perks, and that reason is content itself. And again — at least 90% of registrated users never participate. If we count lurkers that percentage is really much higher for registrations.
This isn’t technical issue. This is much deeper. Most of admins thinks that showing popups, where is begging of joining to mailinglist, middle of reading is a wonderful idea. But it is not. Everyone hates those, even those admins when they are visiting somewhere else.
It is actually just a form of spamming, but with consent. And majority never read those emails.
Same thing with CTA of registrations. It just not matter if a lurker registrare or not. They don’t participate.
But technical limitations of mobiles are obvious. There are not enough space. And if a forum shows ads too… that is really messy for visitors.
Showing registration/login is enough. Out there isn’t absolutely single person who wouldn’t know meaning of those two links.