See, to me, things like this fail on multiple levels.
First, it isnt very significsnt. Tier 1 and 2 it pays off 1 off-save in 20 mostly
Second, since it's just baked in automatically, its invisible. It's just another pre- figured bonys among bonuses.
Three, it reduces character differences. Its literally everyone gets it **and** it drops the gap between the individual choices a tad more. Blah. I want more fidtinctiveness, not less.
Do, I would choose to allow this not as a matter of auto- math but instead add in some form of choice driven mechanic.
Go look at Success at cost in the DMG. Whrn you fail a save by 1-2 (pretty close to the range where half-prof would matter) you can **choose** to succeed but with some on the fly specific personal situational setback.
Which is more **fun ** or awesome to you. ?
Looking down at a +5 save snd making your DC 14 cuz you rolled a 10...
Or
Looking down at your +3 save, rolling a 10, getting 13 vs 14 and saying "wait, though, my guy "does" power thru the webs anyway but it wrenches his pack and dome other gear off - they remain stuck in the webs or stuck together by web strands that tore loose eith us"
Really, at half-prof, the "it matters" cases until you get to +6 at highest tiers falls within the success at cost and turns the "squeak by" into an active and interactive thing eith vonsewurnces rather than just the same toll add compare you do time and again.
Obviously, many other ways to make the "awesome" more than just same- math-different-day.