As far as I can tell, no pure dsl-modem exists anymore, just some routers that allow to hide their routing capabilities... but you can obviously operate a modem-router as bridged modem...
Incidentally that is what I do, run an fritzbox 7520* under OpenWrt as bridged-modem in front of my primary router (turris omnia). This allows me to position the modem next to the telephony socket (german TAE in my case) and the router in the living room, for better WiFi coverage where that is desired. I understand that you do not look for a modem-router, but what I am saying is you do not need to operate a modem-router as your primary router but turn it into a bridged-modem, which seems to be what you are after.
That 7520 bridged modem consumes ~6 Wh/h, see here.
As @krazeh writes there is really no need to run OpenWrt on a modem... I do this because FritzOS does not really offer a bridged-modem mode and I already have a primary router I am happy with.
*) Some selected models of the FB 7520/7530 (without AX) are siemens/lantiq/intel/maxlinear vrx518 devices that should allow profile 35b as well (not tested myself, while 35b is available I am satisfied with the cheaper 100/40 plan). Second hand 7520 often sell for between 50-60 EUR... for installation instructions see here:
https://openwrt.org/toh/avm/avm_fritz_box_7530
Sidenote: in addition to the bridged-modem I use a stand-alone SIP/DECT basestation behind my roputer for telephony, I bought a gigaset more than a decade ago, but nowadays I would probably try to press a second hand fritzbox into that service as well.
There is a note at the bottom of the toh_modem_supported page ( * Last modified: 2024/02/09 12:48 * by 127.0.0.) which might be part of what you are looking for
P.S.: If this is not what you are looking for just ignore this post.