Is a forward slip considered a "normal maneuver" under 91.175(c)(1)?
I can't find any specific definition of &quot;normal maneuvers&quot;, probably because it varies a lot by aircraft type and situation. But I did find AC 120-29A - Criteria for Approval of Category I and Category II Weather Minima for Approach that gives some indication of what the FAA considers not normal (section 4.3.1.6):

Part 91, section 91.175 requires that approach procedures should be
based on use of “normal maneuvers” before and after passing DA(H) or
MDA(H). Normal maneuvers typically do not involve use of bank angles
greater than 30 degrees, pitch attitudes in excess of 25 degrees nose
up or 10 degrees nose down, or sink rates in excess of 1100 ft. per
minute below 500 ft. HAT while maneuvering to land within the
touchdown zone, during go-around, or during a rejected landing. During
a missed approach, pitch attitudes in excess of +30 degrees or bank
angles greater than 30 degrees would typically be considered
excessive.

But even if there is a specific definition of normal maneuvers out there somewhere, I think the context is important too. 91.175 is about instrument approaches, not general flying. So I think you have to ask if slipping to land is a normal maneuver during an instrument approach and I'd say no.
The entire point of an instrument approach is to get you to the runway safely and predictably, and there's a lot of emphasis on stable approaches. I can't think of any obvious reason why you'd need a forward slip if you've flown an approach successfully. You might be able to come up with some scenarios (circling to land? shooting an approach with an MDA of more than 2000' AGL?) however I'm not convinced that it's a normal technique in the specific context of instrument approaches.
If you look at what the FAA says about forward slips in the Airplane Flying Handbook Chapter 8 it strongly suggests that slips are for limited and unusual situations:

Intentional slips are especially useful in forced landings and in
situations where obstacles must be cleared during approaches to
confined areas. A slip can also be used as an emergency means of
rapidly reducing airspeed in situations where wing flaps are
inoperative or not installed.

I find it hard to see how those examples fit with executing an instrument approach. But that's just my opinion. If you happen to know a DPE it would be a great question to ask them.