Any link that ends up in a browser address bar should be treated as public.
And no, it doesn't matter if you use HTTPS. Ways to leak it are many, but the gist is that it's treated as "meta-data" and, rightly or wrongly, subject to much lower expectations of privacy. A recent scandal was that several common browser extensions collect this meta-data and sell it to marketing companies.
Which marketing companies offer searchable subscriptions (for hefty prices, true, this isn't a $9.99 service), where somebody could for example search for "yourcompany.com", or even worse, "yourcompany.com/authernticate?token=". Yeah, meta-data.