The first year of production of IFS coaches (late 1996 & 1997), it was an option. There are still some coaches that are 1997 chassis that have I-beam front suspension. Take a look to verify.
Y-7040 was the first Prevost shell that was an XL2, except for a few prototype coaches for sliderooms.
Prevost did not officially start shipping out their "factory slide" coaches for almost another year (2001 model shells).
Prevost has never installed slideouts on 40-ft shells, but did work with HWH Corporation in the 1990's and Valid Manufacturing in the mid 2000's as they installed slides.
There are some rivetted XL's with slides, primarily Vantares or Marathons.
Vantare, after being purchased by Featherlite trailers, began their slide program using HWH hydraulics under the engineering and direction of the Cresco, Iowa Featherlite headquarters.
Marathon began their slide program around the same time with Rogue, another Oregon company, and later HWH, followed by Valid Manufacturing in the 2000's.
Ben Cummings
U.S.A. Luxury Coaches, LLC
Clearwater, FL