https://youtu.be/UWhs8Hc65_Q
Start at 16 minutes to hear them talking about guardian. Short answer is it's not nerf cappy and done.
Not sure where this is going to all land since we haven't seen the results of lore/disable nerf + the planned boss damage adjustments. I can say reports from one T5 raidgroup I know in this past week of Bullroarer--with lore nerfs but no boss adjustment, making mid tiers not feel so farm mode--indicated Shelob spike damage would take (near) invulnerability cooldowns like old SotD and LS to survive through. Which a guard does not have; Juggernaut and Pledge help against autoattacks but not boss special attacks. Also as it was on BR (pre damage pass from Vastin) guards have no answer to the Daughter's jaw cleave (no worthy one incl redirect). By another token, the same people were tentatively thinking guard for Durin's Bane.
We dont know what live U30 will look like yet, whether things will be easier or the same or whether farm content is retroactively made progression again, so I can only speak in broad strokes: Yellow cappies are going to remain a lock in 2-tank comps because shield bro and unselfish SotD. As it always has, boss spike damage will favor choosing tanks with big working cooldowns and that includes nerfed captanks with just LS and beornings (hot take: best tank for Thoss now) but does NOT include guards. And high sustained over spike damage favors guards with their superior passives and their increased relative support (since some but crucially not all captank support has been made redundant).
Argendauss, Captain
Rechart, Warden
Hrodgart, Beorning
Gunnart, Guardian