e.at() is not a function

[steffenkleinle]

Describe the Bug

We have lots of issues of e.at() is not a function in sentry:
Sentry Issue: INTEGREAT-APP-70V

This seems to be related to deepmerge and missing support for Array.prototype.at() in old browsers:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/at#browser_compatibility

It might cause requests to fail.

Steps to Reproduce

Reproduction needed.

Expected Behavior

I'd expect everything to work.

Actual Behavior

Additional Information

Investigate why this is happening and why our es-check tool does not detect it.
A polyfill might fix this issue.