It appears based on the contents of https://support.zabbix.com/browse/ZBX-23900?jql=text%20~%20%22tag%22 that you are trying to nudge  users to in general use tags that follow the template conventions ( https://www.zabbix.com/documentation/guidelines/en/thosts/configuration/templates#tag-name-and-value-format ) everywhere in zabbix. It also seems like based on [ZBXNEXT-8772] Support tag.name and tag.value in aggregate calculations - ZABBIX SUPPORT that having colons in tag names and values causes filtering issues.  

If you are indeed trying to get everyone to use template compliant tag names and values please update the web interface, so that when someone tries to use a Tag name or value that does not comply with the guidelines that it complains with a message along the lines of "A tag name or value does not comply with the zabbix template tag name and value guidelines. You should consider using tags that comply with those guidelines should you wish to in the future add this tag to a template." (possibly also include link to a new tab that takes you to the guidelines or what the guidelines are.)  Then to allow users to continue to use older tags that don't comply or just be "dangerous"  with their tag choice, by giving the user the options to go back to editing the item to make it comply or to use the noncompliant tags anyway. 

Why: In response to a recommendation of zabbix support in an enterprise ticket trigger actions I went through multiple manual pages and ended up at the Template tag name and value guidelines (not realizing I was in guidelines for templates only) and found out I was using a not allowed in templates colon and that the zabbix engagement staffer used both uppercase characters and a forward slash in discovery actions.  This led me to open a support case about the tag naming violations, what the potential ramifications are and how to in bulk deal with it as we had used these non-compliant tags on a lot of systems. I was told it is fine if it isn't being used for templates and that max length is basically all the needs to be worried about for tags not being used in templates.  However I would think that if it is not fine for a template that users should at least be warned that it is not best practices and that they should consider following best practices should they eventually want/need to use the tag in template, so that I don't create and propagate new non-compliant tag all over the place and then have to figure out where all it was used and change the pre-existing stuff to make a tag that works in a template.

Also the way https://support.zabbix.com/browse/ZBX-23900?jql=text%20~%20%22tag%22   documentation changes ended up making things look this caused me to wondering if our zabbix engagement had done things in an acceptable manor, and ended up wasting my time by having me jump to conclusions and go in and modify the stuff I just set up to be lower case without special characters from the tag names and values and internal documentation, and additionally waisted your support staffs time due to me unnecessarily opening a support ticket about all the tags values that did not comply with the template tag guidelines  

Note another option on how to deal with this is to allow the Use of more characters in templates (though it was mentioned in the ticket that the reason for less characters is an attempt to standardize template syntax.)