Yiisoft DI package requires PHP 8.0 now, it seems. 
SlimPHP v4 requires PHP 7.4, according to their docs. Installation - Slim Framework
Yiisoft DI package requires PHP 8.0 now, it seems.
SlimPHP v4 requires PHP 7.4, according to their docs. Installation - Slim Framework
What are you trying to say?
That I’m forced to use SlimPHP 
Did you try to upgrade to 8.0 and see how many things break?
Unless you enjoy SlimPHP, I would try to update my PHP version in test environment and see
Highly sensitive and sadly understaffed project, not possible until next year at least, which is too late.
I mean, you just make a copy and upgrade PHP version and see how it works. One time I was surprised that I had very few things to tweak and make it work with PHP8.3. But that was Yii2. You might want to give it a go!