In Dynamics 365 HTML web resources, we often added the global context by including the ClientGlobalContext.js.aspx.
As this ClientGlobalContext.js.aspx is built on the legacy web client infrastructure, it is deprecated now and won't be accessible after 1. December 2020.
So, it's time to replace this old feature now!
If you're using a HTML web resource within a form, you're lucky. You simply can call the Xrm object from the parent.
If your'e using the HTML web resource outside a form (e.g. from the sitemap), you'll have to do a lot more to replace the ClientGlobalContext.js.aspx. Basically, you'll need to use the REST api to interact with the system.