5.6 previous change UI to previous UI

in v5.6 i changed

<entry key='web.path'>./modern</entry>

to

<entry key='web.path'>./web</entry>

after copying web folder from v4.15

just curious ...will it cause any problem later??
if yes what will those problem be??

Why use the web app from 4.15 instead of the legacy web app from 5.6?

looks like ./web and ./legacy is same
is it??

Almost, but the legacy folder is kept up to date.