This is a general curiosity.
CodeMirror 5 is written to be compatible with IE9 (released 2011).
IE11 (2013) was later dropped and replaced with Edge (2015).
The entire IE market share is 1.09%
IE9 market share is 0.21%
How long/far the backward compatibly should be kept for outdated/hardly-used browsers?
Modern JavaScript has had many improvements in performance, stability & usability which the great majority who are users of modern browsers are missing out of.
reference: https://caniuse.com/usage-table