So each cult has strictures, which are mix a small fluffy requirements to more restrictive requirements. There is no alignment in Warhammer, so the strictures are more related to the god's goals and area of dominion. Some strictures are very black and white and easy to determine whether you broke it or not ("never take up arms, a walking stick and courage will suffice") whereas other are open to much interpretation ("Shallya's work is never done, so turn not to self-indulgence."). The strictures vary greatly from one cult to another, so there are plenty of options for players to find one that works for them and, as a DM, I am fine with working together with the player to homebrew a cult and its strictures. When a character violates a stricture they will gain a number of sin points depending on the scale of the infraction (normally 1-3 sin points). Characters in careers that offer one or both of the Bless and Invoke talents, and the Pray skill, can call upon their god to enact a Blessing or Miracle. If they succeed in a pray test, the blessing or miracle occurs. If they roll well, they may have additional success levels that will give bonus effects. But, if your prayer roll results in a number less than or equal to your sin points, you suffer a roll on The Wrath of the Gods table. It usually results in a mechanical debuff or negative condition, but a very poor roll can lead to you losing the bless and invoke talents.
That's where penances come in. These are activities that can remove sin points. They can be things that you do on your downtime or they could be the subject of an entire side quest or adventure.
It is a bit more crunch than D&D, but not much more than a 5e wild magic sorcerer with its wild magic table.
The way I've brought this concept to 5e is that I'll come up with some strictures or penances for clerics, warlocks, and paladins. Often that isn't hard to do, as some settings like Forgotten Realms already have a lot of this written into the lore. I'm happy to create this with the player if they are into that. Instead of sin points, in my last campaign I worked it into the renown, infamy system and also used the concordance system from MCDMs Strongholds and Followers book. So if the PC isn't acting in accordance with the tenants and social norms of their religious order or cult, the negative consequences were mostly social. They would lose Concordance for the extra-special abilities it offered, but they wouldn't lose their class or ability to cast spells. My in-world reasoning for this was that maybe their god or patron was unhappy with them, but another entity may step in to given the character power and now the character is actually unwittingly acting in the interests of another power. A bit sloppy, but it worked and we had fun.
But now that I've been playing Warhammer, I would seriously consider bringing the idea of strictures, penances, sin points, and wrath of the gods to my D&D game. But only if the players were into it. Also, I alway play new rules RAW for a while before adding homebrew, so I expect I'll run the 2024 rules RAW in a first shorter campaign before discussing any changes or additions.