Having a mobile optimised website is vital for attaining a good ranking position. Google is highly favourable to responsive websites and gives a ranking boost to all websites that are mobile optimised.
Google contends that 57% of search is now conducted on a mobile device. A mobile-friendly website ensures your users have the best possible experience.
Google introduced its Mobile First Indexing in March 2018. Although it is unclear when the Site has been moved to Mobile First Indexing it is highly likely that the migration has been completed by now.