Sanctions in situations like this typically apply only to government owned or operated entities; citizens and private individuals are usually exempt -- unless they (or a controlling owner/operator) are specifically identified in a set of sanctions or are listed in the US "Specially Designated Nationals And Blocked Persons List".
In the past, ISRG/LetsEncrypt have blocked certificates for domains connected to sanctioned governments and specially designated individuals, but still allowed independent citizens of those countries and owners of CC TLD domains to procure certificates.
I think we can expect ISRG to begin blocking issuance to that limited subset of applicable entities/domains - if they haven't already.
However, the blockage would assume those entities/domains utilize LetsEncrypt Certificates to start. Perhaps some of the individually sanctioned oligarchs own companies that utilize LetsEncrypt certificates, but SSL/TLS in Russia is pretty complicated. Putin and the KGB have been exploring making SSL Certificates illegal, or requiring only certs from a state run CA for a few years (I'm not sure on the status of that now, but they were publicly looking to join the CA/B forum and root programs a few years ago). I've read in the past that a lot of public SSL stuff in Russia is based on side-loaded roots, because the promoted/preferred CAs are not pre-loaded into trust roots.
TLDR; ISRG will have to limit, but the actual impact will be minimal if anything.