If your client is based overseas with no presence in the UK, then the IR35 rules don’t apply. If they are based overseas but do have a connection to the UK, then they have to assess your IR35 status.
If your client is a medium sized/large sized non-public sector company based overseas and they have no UK connection (i.e. no branch, office or subsidiary here) then the IR35 rules don’t apply. If you are working for them through your PSC, you can continue to self-assess your IR35 status.
However, if your client is based overseas and does have a connection here, then they need to assess you for IR35 and provide you and your agency with a status determination statement.
Your client must do this if you are subject to UK tax – if you are UK resident and/or you perform your duties in the UK. If you are UK resident and perform your duties overseas, then you are still subject to UK tax, and the engagement is still within IR35 rules for you and your agency. If you are UK non-resident and perform your services overseas, then the IR35 legislation won’t apply because you are outside the scope of UK tax.