Switch the API with the button indicating "OA / S2 / CR" in the top-right menu.
OpenAlex (OA) is used as the default as the direct successor of Microsoft Academic, which was discontinued in 01/2022. It is fast, it has high coverage and serves abstracts. It also serves a flag for retracted papers, which can be seen here for example (it can be missing though).
Semantic Scholar (S2) also has high coverage, also serves abstracts and allows accurate estimation of the reference-list completeness. In addition, it easily serves incoming citations for each paper, allowing the generation of outgoing suggestions, which are newer than the input articles. However, it can be slow and has a rate limit of 100 requests (i.e. papers) per 5 minutes per user, limiting its use.
Crossref (CR) has the advantage that it usually returns reference lists in their original order (alternatively you can use the bookmarklet). Also, it allows for an accurate estimation of the reference-list completeness, it's usually very fast and can handle fairly large input lists. Crossref is a not-for-profit organization that collects metadata from scholarly journals and publications. Thanks to the formidable Initiative for Open Citations (I4OC), the coverage of Crossref has increased vastly. However, some journals still do not support these ideals - feel free to write them, in case you find one!
The metadata API from OpenCitations (OC) theoretically also features outgoing suggestions. However, it can be slow and unstable, which is why it is deactivated by default. Activate it here experimentally, but I advise to only try very small queries.