I figure there might be a difference between what they can do in their home realm, and what they can do elsewhere.
In their home realm, I'd say that any rank of deity should have the abilities of a 10th-level cleric, with any appropriate domains. I'd also grant them leeway to cast pretty much any lower level (including cantrips) or non-combat spell that seemed thematically appropriate without feeling a need to write it down as an ability first. This would all be in addition to the kinds of special abilities that official 5e statblocks might include.
I also say they can just straight up cause non-spell effects that fit within their portfolio while within their home realm.
Any non-deity capable of empowering warlocks should have the abilities of a 10th-level warlock of the appropriate pact when in their home domain.
This allows for deities that have a particular emphasis in a type of magic to excel by being better than 10th-level at it.
When outside of their home domain, I'd probably limit their capabilities to what is in their statblocks, plus the free thematic low-level and non-combat spells.
That's my way of balancing making things make sense with sticking as close to the written rules as I can justify.
It's worth noting that if you don't grant all deities and warlock patrons access to 20th level class abilities, you are saying that they can grant or teach abilities that they don't possess OR that some of the abilities aren't directly "granted". That's fine for me, because I take the latter approach in any event. Divine power makes you a cleric, and after that you don't need any direct agency from the deity to do your thing. They (probably through an intermediary lineage of priests) just grant you access to the same source of power they themselves have, and might keep an eye on you and punish you if you misuse it in their name (depending on the deity). Same basic concept with warlock patrons. It's Poof You're a warlock!, and now you can level up all by yourself. Any actual agreement is something that must be enforced by the patron actively enforcing it--they can't just take away your powers, and if you're high enough level you might even have powers they don't! (Though it's rather unlikely you are going to individually be more powerful than them overall, given the write up of arch-fiends in 5e.)