This is a really unfortunate change resulting in a loss of important functionality. Hopefully it is just an oversight and it can be fixed to open in the user’s “edit observations” page with the correct records queried out. I have emailed iNaturalist support about this.
In the meantime, with some knowledge of iNaturalist search urls (see https://forum.inaturalist.org/t/how-to-use-inaturalists-search-urls-wiki/63) it’s possible to take the search parameters from the explore page that opens up and use it on the edit observations page to query out suitable observations, then use the “batch edit” option to select all and add them to the project. As an example, here’s it in action for a project I help administer: https://inaturalist.ca/projects/nhic-rare-species-of-ontario/journal/9226-what-records-are-eligible-and-how-can-i-quickly-add-them-to-the-project