Compared to older model MBA (2017 and prior) -- the added cover would add extra weight on the screen portion. When I grab/hold and lift my MBA (while it was OPEN, older model), the lid/screen would carry some "momentum" and would "extend" the screen to fully open position. If you use the MBA on the lap, with tilting downwards, you might notice the screen will slowly extend due to this weight. This will affect the hinge if I am not careful with this. In the mean time, the hard case would decrease the heat ventilation (Intel MBAs have fan for cooling).
However, I still used the case, mainly to add extra grip-security due to my sweaty hands.
M1 MacBook Air - there is no fan and thermal is quite impressive. The hinges on M1 MBA are much tighter than 2017 MBA and prior. I do not use MBA for heavy workload (mainly for web/document and web-conferences) and continue to be vigilant about the hinge.
There are many vendors on the internet, like Amazon..etc. carry MBA case (some even comes with keyboard cover).
I got Mosiso cover for about 17 USD.