Titles work so long as the characters belong to, return to, and further the ends of, an organisation that bestows them.
Adventurers are by their nature peripatetic - the return to base to demonstrate the skill/power increases gained via the XP mechanic would need to be a regular thing.
It works nicely for Wizards' schools and Cleric's churches; also for Thieves' Guilds (assuming they are willing to be registered...and if not, adventure hooks); Bards' colleges; Monks and Paladins (see Clerics), Druids (could be a regular 'moot' of the Circle); a certain type of very formalised military fighter, too perhaps; less so for Sorcerers and Warlocks, Barbarians, and Rangers (Rangers could be affiliated to a Druid moot perhaps).
Titles could be bestowed by an organisation such as the Harpers, though they have their own titles which are based on 'what have you done for us' rather than XP accumulation.
Having titles granted would mean a formal return to the awarding body which feels like an RP opportunity rather than a hand waved downtime activity. So for me, it lends itself more to the 'tier' system so that it is a less common, less mundane, more of a big deal thing every, what, 4 levels or so til level 12.
Further title advancement entirely down to engaging in old school 'name level' activity - building a keep/church/tower etc...
So, eg:
Wizards: Apprentice - School name eg Evoker, Enchanter, Seer - Mage - Magus (I love Magister but feel it's head of the order territory)
Clerics: Acolyte - Priest - then High Priest then something deity-specific eg Stormbringer, Great Skull, whatever)
Rogues: Prentice - Brother/Sister - Master - Grandmaster
Not great I know