The supertele lenses all used to have a meniscus lens in front (essentially a flat piece of glass to protect the first refractive element, a permanent clear front filter). Dropping those from the design was a significant part of the weight saving for both Canon and Nikon lenses.
The non-IS Super Telephotos had a flat plate. They were designed in the pre-digital era when reflection off the front of perfectly flat filters in front of digital sensors was not an issue since film was still used in all EOS cameras in 1996 and earlier.
The almost flat plates which were ever so slightly meniscus to avoid reflections from the front of a sensor stack bouncing off the back of the protective plate only appeared on the front of the original IS Super Telephotos introduced in 1999. On the spec
sheets, Canon still called them a protection glass. In 2004 Chuck Westfall gave the following list of Canon lenses with the protective meniscus:
EF300mm f/2.8L IS USM
EF400mm f/2.8L IS USM
EF400mm f/4 DO IS USM
EF500mm f/4L IS USM
EF600mm f/4L IS USM
Two more were added in 2008: the EF 800mm f/5.6 L IS and EF 200mm f/2 L IS.
The IS II series introduced in 2011-12 lacked the almost flat plate.
The major shift of weight to the rear and the resulting advantage of smaller elements began with the EF 400mm f/2.8 L IS III and EF 600mm f/4 L IS III in 2018.
Note that the EF 300mm f/2.8 L IS II, EF 400mm f/2.8 L IS II, EF 500mm f/4 L IS II, and EF 600mm f/4 L IS II, introduced in 2011-2012, other than removing the almost flat cover plate while also using fluorite for the second element as well as keeping the fluorite fourth element (fifth element in the older lenses with a protective plate), did not shift much of their optical formulae rearward. Their weight savings, which did not approach anywhere near the same degree of reduction as with the 2018 EF 400/2.8 IS III and EF 600/4 IS III, were as much a result of reducing the weight of non-optical components throughout the lens as they were of changes made to the optical formulae.
The 2011 EF 400mm f/2.8 L IS II (The 2011 EF 300mm f/2.8 L IS II was very similar):
Blueberry is fluorite
The 2018 EF 400mm f/2.8 L IS III:
Green is UD, purple is fluorite
The 1988 EF 600mm f/4 L:
Green is UD, purple is fluorite
The 1999 EF 600mm f/4 L IS:
Blue is UD, pink is fluorite
The 2012 EF 600mm f/4 L IS II (The EF 500mm f/4 L IS II was very similar):
Purple is fluorite
The 2018 EF 600mm f/4 L IS III:
Green is UD, purple is fluorite