Steve, come on man. You know as well as I do that these echoes are really not supposed to see the light of day. I don't know what this code is you are looking at, but just by reading it you can see that it is not for a user interface, more likely some kind of status message. All of it is located in the admin or common sections, not customer facing.
Some of the <font> tags that I can find are from
1. Bootstrap
2. TinyMCE
3. Doctrine
4. FontAwesome
The ones that actually are core XC5 code and not some plugin or lib, are all error messages and warning messages. Most of them are dealing with the installation process, before CSS files and the like even properly exist. I found 40 occurrences across 17 files.
I personally am much more worried about the dreaded <table> tag. I found 205 matches in 117 files. I hate tables. They should all be destroyed!
But saying XC5 uses deperecated code is a little harsh, and a little wrong. I admit, yes there is the now deprecated font tag, proof right there in the pudding. A deprecated tag that is still supported by every browser in existence. I doubt that browsers will every really drop support for the font tag. What they need to do, is drop support for those pesky tables!