Since Jira 10, it is no longer possible to simply update the JSP and have customisations applied immediately at runtime. This has been disabled for security reasons according to Jira Software 10.0.x upgrade notes

Disabling the runtime JavaServer Pages compilation

JavaServer Pages (JSP) runtime compilation will be disabled in Jira Software 10.0. JSP files added to the Tomcat directory that aren't shipped with the product won’t be served. Furthermore, no modifications to the JSP files will be reflected. We recommend using Soy or Velocity templates instead.

and to customise Jira JSP pages,  Customer will need to build them from the source as described in the article: Precompiling JSP pages. which is make it a bit complex to the customer andd for that this Feature request has been create to allow update the JSP and have customisations applied immediately at runtime.