Shield Bash is a really bad rune. It's super niche in terms of tanks who can actually use it without weird itemisation or the Guardian rune.
Tanks who can use shield bash:
(kinda)
(kinda)
(kinda)

Tanks who can't use shield bash:

Being realistic, Blitz, Galio and Malph have too little control over their shields for this to be a viable rune on them, they can just technically make use of it not practically make use of it. So under half of tanks can actually use this rune at all. I was also generous as to what constitutes a tank in list 1 and harsh in list 2. The actual pool of tanks who would even look at this rune is J4, Naut, Ornn, Shen and Sion. The others all have better choices.
Having a niche rune isn't always a bad thing, but when the other options are Font of Life, a rune that's already niche and Demolish, a rune that you take because you have no other options and don't generally want it's a bad idea. If anything this rune should have replaced Demolish, although I still don't think it'd be a good idea.