Why have capacitors improved so much over the years?

To me it’s definitely materials and new manufacturing techniques. I have Duelund cast silver caps in my 1970’s luxman amp with Teflon caps and sounds absolutely stunning and has got me off the amplifier merry go round. They really are amazing caps. My tech is also part of the stunning sound this is important for your upgrades too.

I didn’t realize that any crossover destroys the signal.

Not all. Time correction can be done, but it requires thinking "outside the box"

Nearly 50 years ago Dahlquist did the DQ-10. Nearly 40, John Bau did the Spica TC-50 and later the Angelus. Also Wilson, Van der Steen, ... All the crossovers were complex and none of them had drivers mounted on a flat panel.

No passive crossover in a flat panel box will ever equal equal phase aligned drivers each driven by a suitable amplifier in turn driven from a bespoke active crossover. To think otherwise is kidding yourself.

The new ODAM caps from VH Labs
are quite amazing besting even cast copper Duelund.
in comparison they made the Duelund sound slow
and not as open and organic as the VH labs.

Where are they used?

Did you verify values?

Please detail evaluation.

Please note that you appear to be a dealer of some sort:
"As a dealer I do realize the challenge it presents when you attempt to set up a high end system in a store environment. That is why our high end reference system is actually set up in a home environment." in  Audiogon Discussion Forum

For high voltages, 300 and higher including 1000 or more, I feel safer using polypropylene to electrolytic because of shelf life issues the former do not have and you can get polypropylene good for 1300 Volts for about the same price you would have to pay for banks of electrolytic capacitors wired in series and parallel to get rated 1800 Volts. The former are larger, 3" diameter but you can get an aluminum box 17" by 17" that will have enough room for two power supplied with chokes on the negative sides. I use fast solid state diodes with center taps on power supply transformers rather then risk the problems with high maintenance mercury vapor diodes and the isolation transformers for their cathodes that have to be pre-heated and could short circuit.