Old thread I know but I've just recessed my master socket as illustrated in the first post. It was on a 25mm surface mount box and protruded stupidly.
It was always a bit sensitive to "activity" nearby so I also wanted to rewire, so I actually also stripped back a bit of the incoming cable; a bit even snapped off during the process so it was obviously a bit fragile/damaged already.

I also verified the voltages and found that And B were the wrong way round - well done the last BT engineer (who replaced my drop wire).
Interestingly it's now very stable, slightly faster and totally insensitive to the vacuum cleaner etc.
Anyway, to the point. I'm planning to move to FTTC, but by initially installing a new line, which will be in a more convenient location (assuming BTOR don't get stroppy!). This copper line will then be reduced to voice only and eventually retired.
My plan for it "cosmetically" is to revert to an older-style near-flush BT master socket. I'm sure I've got one somewhere and if I need a recess it's already there.
I'll leave it like that even when it's totally decommissioned. As @Chrysalis said "for good old memories". And to confuse the younger generation.

I'm almost tempted to revert it back to what we found when we moved here, back in the days of Post Office Telecommunications. There was a Plan "something" setup of a main phone and an extension phone connected via an old school big jack plug and socket, all rented. That got changed pretty quickly and has been through many iterations since, including at one stage (pre-DECT) lots of wired extensions.