Mercy has been reworked to have Rez on E, and Valkyrie as her new ultimate. She still retians the nerfs she’s had since the Moth meta, with a few reverts to her healing and some minor QOL fixes such as the lowered cast on rez while using her ultimate.
Mass rez does exist in the Workshop however, thanks to the creativity of the community, and even making their own tweaked versions of the hero. I’m even working on a build of Mass Rez with tweaks as we speak. It’s ever evolving and improving, and you are free to play with the builds at your leisure. 
Many people have been playing custom made Mass Rez versions of Mercy on PTR, and you can easily find examples of the amazing things they’ve made on social media / youtube / etc.
So in short, is Mass Rez still gone? In terms of the main game modes (QP/Comp/etc.) I’d say it is for the moment. In general however, no. It still lives on and has become quite popular in the workshop scene. 
~Sincerely Yours xoxo,
a Lover of True, Fair, and Fun Balance.
xavvypls 