It's a method useful for startups only.
Businesses who have moved on from startup status need to switch to a quality first approach.
Deprecation should only be on the table if your original design was flawed. And then only if you are certain you are fixing that particular design flaw completely. So many times have I seen rewrites due to a poor design just create the same design flaw in the new system as a compromise because changing the database would be too hard.
And even then, I strongly question the need for isolation/deprecation like this. It says a lot about your company and creates a broken windows atmosphere where it's ok to push shit because it will eventually be replaced anyway.
"Wanting typescript" is a shitty excuse for putting a business critical system up for adoption.
Literally nobody will want to deal with it within a year. After that, your business critical function, likely one that makes money, is no longer maintained and impossible to change.
Enjoy your growth stagnation.