You cannot activate shields while standing on scaffolding. Since sneaking/crouching is the only way to activate shields, you cannot use them while on top of scaffolding since there is no button for shields to be activated whatsoever. Mobs could attack you while you are building, meaning that you are safe.
MCPE-45810
You cannot activate shields while standing on scaffolding
Resolved
Works As Intended
0
(Unassigned)
165727, 168948
Confirmed
Android
1.13.0.1 Beta, 1.12.0.13 Beta, 1.12.0.11 Beta, 1.12.0.10 Beta, 1.12.0.3 Beta, ..., 1.11.2, 1.11.3, 1.11.4, 1.13.0.2 Beta, 1.12.0
Retrieved
Thank you for your report!
However, this issue has been closed as Working as Intended.
If you need help or have a suggestion you might like to follow a link below.
Quick Links:
📓 Issue Guidelines – 💬 Mojang Support – 📧 Suggestions – 📖 Minecraft Wiki