Hello, author of PVPTooltips here.
Addons have now access to instanced stat templates (since patch 7.2 IIRC), so someone could pull that data and present it on a website for example, it
is feasible.
For spell modifiers however, there is no easy way to find out how spells are modified in PVP without reading all hotfix notes since the release of Legion... That's basically how I update spells when I want to release a new version of my addon: read hotfix notes and hope Blizzard is accurate and hasn't forgotten anything (which is ... well LOL
)
When PVP rules are enabled, Blizzard spell tooltips automatically change to take PVP nerf/buff in account, but it is incomplete. Some spells that are modified in PVP do not list correct values (such as Bear Form granting less armor in PVP). So basically, it sucks all around.