My 1986 Newport 28 MKII has a slanted fiberglass
cabin sole and it does not flex. If the one you are considering has a flat teak
and holley floor, it could be rotten due to water leaks
or the cabin sole was under water for a while. I would lift
access panel in the cabin sole and look/feel around the underside of the sole for rotten wood or loose cabin sole supports. The supports could have rotted. I have been on several N28's with the teak
and holley cabin sole and they were all solid, no flex. The bigger issue is did a lot of water get into the boat from a partial sinking, etc. Look at the bulkheads (vertical wood walls)mast support and lower cabinet doors for staining from standing water that might have covered the sole. Probably worth having a surveyor
look at it if you are not sure about what you find. Feel free to PM me if you want more info in the N28.