Prepostions followed by that-clauses

I don't entirely agree with the OP's proposition:

prepositions take declarative clauses without that

For example, you can't omit it without rendering 2 ungrammatical:

2'. *You can't always count on (that) someone will help.

Note that 2' is ungrammatical with or without that.

Thus, some prepositions such as on cannot take declarative clauses with or without that.

Having said that, I believe examples of a preposition taking a that-clause as complement are few and far between. Most of these rare examples are not even legitimate examples, if you will.

In 1, for example, the preposition except taking a that-clause as complement can be better explained by ellipsis, as shown below, than by considering it a legitimate example of a preposition taking a that-clause as complement.

1. The apartment is nice except (for the fact) that the rent is too high.