Its unlikely the lens would be compatible. For a variety of reasons.
Lenses manufactured before 2008 do not support DPAF, have slow motors, and even if capable of being adapted may not include communication protocols needed for the lens to properly function and interact with an R series body. Canon does make an accessory for mounting EF / EF-S lenses to R series bodies, but just because something fits doesn't mean it will function properly.
You would not be doing yourself or your new camera any justice. In this case, I would recommend a new lens, or a lens that was new enough to take advantage of the R series capabilities. If you tell us which, we can help confirm this for you.
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Yes EF & EF-S lenses can be adapted. BUT older EF & EF-S lenses lack FULL COMPATIBILTY with the EOS R series. This is NOT ONLY true for the EOS R series it is also true for DSLRs that support DPAF. Older lenses simply lack the hardware to take full advantage DPAF. This is because the lenses were DESIGNED for stills use NOT video. So don't expect adapting an older lens will ALLOW FULL USE of ALL camera features. You'll be RESTRICTED by the lens' hardware NOT SOFTWARE. Also a lot of these lenses use older slower AF Motors such as AFD (Arc-Form Drive 1987-1992 replaced by Micro Motor), Micro Motor (1993-2012 replaced by STM) & Micro Motor USM (1993-2016 replaced by Nano USM). Ring Type USM lenses released prior to 2009 ALSO DO NOT SUPPORT DPAF they will often "snap" back and forth to acquire focus. Newer Ring Type USM lenses support slow focus transition and apertures that stop down in steps. Instead of jumping from setting to setting.
No they WILL NOT mount directly to the EOS R100. The EOS R100 uses a new lens mount called the RF Mount. Your Digital Rebel uses the EF & EF-S Mount lenses. Your lenses CAN BE adapted to the EOS R100 via an EF-RF Mount adapter from Canon. Also older lenses ARE NOT compatible with DPAF (Dual Pixel Autofocus). They'll work if adapted but your camera's AF performance WILL BE LIMITED due to the lens' older hardware. I would suggest using lenses released in 2009 OR NEWER. It IS NOT recommended to use a 3rd Party mount adapter. Adapted 3rd Party lenses may be hit and miss when it comes compatibility.
What everybody is saying may or may not be important to you. DPAF is not a requirement for instance. In all likely hood the older lens will work as well as it ever did on your Rebel XTi. So, if you were happy with it then you will probably be happy with it now. Some folks say EF/EF-S lenses work better on the new R series cameras.
09/26/2023: New firmware updates are available.
08/18/2023: Canon EOS R5 C training series is released.
07/31/2023: New firmware updates are available.
05/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
03/30/2023: New firmware updates are available.