SPA is HTML/JS served by the server. We don't need client-only solutions. We need devs to understand how HTTP and browsers work.
It means that we simply keep using what actually works, i.e. serverside component that obtains authorization and we use simple mechanisms to ensure token stays at the server and FE speaks to the server which in turn speaks to the target app. Proxying is not that difficult of a problem and we don't have to run in circles, inventing different flows only to cater to devs who can't learn their field.
We need CDN solutions for front-ends because that's the best way to deliver great, scalable performance for complex SPAs.
We also need a purely client-side flow for mobile (native) apps.
Additionally, the authorization code flow (with PKCE) in Keycloak still supports pure client side authorization. Its more complex than the implicit flow, but it doesn't really matter as any library (including keycloak-js) will take care to ensure its done correctly.
It means that we simply keep using what actually works, i.e. serverside component that obtains authorization and we use simple mechanisms to ensure token stays at the server and FE speaks to the server which in turn speaks to the target app. Proxying is not that difficult of a problem and we don't have to run in circles, inventing different flows only to cater to devs who can't learn their field.