Also possibly relevant here is
this tweet from Jeremy Crawford of WotC, implying that once a warlock has learned powers from their patron they’ll keep them no matter what happens to their relationship. Though this seems to make more sense in the context of a master/pupil type relationship than a deity/disciple one.
This has been said multiple times by different official rules writers, but it's always a pain to find, so thank you for saving me the trouble of finding it.
I hadn't seen that tweet. Still, he doesn't address pact breach necessarily. Now if the powers cannot be revoked the patron is surely not obligated to give more so surely flagrant pact breach would result in inability to level up further into warlock under that pact and potentially the patron even sending people to assassinate your character.
The term "Patron" can be interpreted as something similar to "Supporter".
Even though your patron may not like you, the contract says that he must provide you with the minimum necessary support.
So as long as you're not doing something that directly violates your contract, you're safe.
And even if your patron dies, you can still gain new levels, but some doors will remain closed to you, as some of the warlock's abilities require direct assistance from its patron.
For example, some of the more powerful Warlocks can ask their patron once a day to restore all their spells.
Though the punishment Wyll gets is certainly unique, though I am not sure what it does beyond alter his appearance.
And are you sure it's a punishment? Isn't it possible that it's a reward?
If someone pumps you with devilish power, it's not uncommon for some physical changes to occur that make you look a bit like a devil yourself.
