Mounting slides in Anti-Newton glass keeps them flat for sake of better center to edge projection sharpness, and also protects them somewhat from handling. Plenty of UV and heat will still get through to affect them. The longer they are projected, and how often, determines the lifespan of the dyes, besides how specific films might differ in that respect. But with glass mounts, it's especially important to store the slides in a well ventilated rather than humid area, to prevent fungus on the emulsion. I've used Gepe 6X7 glass mounts, as well as 35mm. I don't think 6x9 mounts were ever made, but could be wrong. Flat-field projection lenses are needed, rather than the usual kind.
Back in the day of professional slide shows, often with very powerful projection lights for sake of big audiences, they often ordered duplicate slides of key images to protect the originals from fading. The quality of lab dupes varied from OK to rather miserable, and was somewhat costly when properly done. But having a relatively pristine original not only allowed the possibility of more duplicates later on, but of potentially making prints for sale from that particular image. The special characteristics of extant duping films allowed this to be done without masking, often using some kind of instant flashing device instead. Masking would have been way too expensive anyway for such volume applications, though I personally prefer it quality-wise. Now the contrast curves are commercially controlled via scanning and software, as in the previous post. Masking is more a "home-cooking" option today. But nuthin' beats real home cookin'.