Can I access Posts of custom-fields via URL?

If you are asking if there is native way then the answer is no. While simpler and older representation of meta queries using meta_key and meta_value would technically fit into URL, they are not registered as public query variables.

Even considering registering them as such would be too dangerous in a blanket way, because that might cause all sorts of private information become accessible.

However, if you are asking if this is doable, then answer is that it’s completely is. You would need to create custom rewrite rule to respond at such URL and limit the processing to specific (and publicly safe) custom fields.