Microsoft has started blocking execution of custom markup in calculated fields in SharePoint Online from June 13, 2017 onwards.

Reference

Workaround

• JS Link Approach – JSLink allows us to change the field formatting based on field values or custom logic. This approach may not work for calculated column and modern concept.
• DOM Manipulation at runtime using a script editor webpart – This is a temporary solution as Microsoft is recommending users to use modern features.
• SharePoint Framework Extension – This approach can be used only when we switch to modern concepts. We can make a ApplicationCustomizers using extension approach to derive a generic solution.

Temporary Fix for Classic Pages – DOM Manipulation at runtime

Sample Formula

Sample JavaScript Code to add in Script Editor WebPart