My problem with using defunct letters is that it requires a lengthy explanation, and even then is still hard to read. It would be easier just being in a different language entirely, at least I could copy and paste into a translator.
ð and þ are lost letters of ð english alphabet which technically were used interchangeably, but in oðer languages represent two distinct sounds.
Þink vs Ðou if you want to sound it out to get what's being distinguished, it's like ð difference between B and P, or D and T, but for ð two sounds you hear whenever you read a th.
Which still doesn't really answer why use them. Why make it intentionally harder to understand your comments? What's the upside to it?
Edit: I've looked at your previous comments and saw you explain it to someone else where they've come from and your preference, but I feel like if you have to explain it to everyone it can't possibly be a useful way of communicating.