I get some trailing whitespaces errors when I try to apply this on my local repo:

pbr: update to 1.2.1-r35 #28037
<stdin>:91: trailing whitespace.
        /etc/init.d/pbr netifd check && {
<stdin>:94: trailing whitespace.
        }
<stdin>:106: trailing whitespace.
        /etc/init.d/pbr netifd check && {
<stdin>:109: trailing whitespace.
        }
<stdin>:566: trailing whitespace.
                        for i in "$nftTempFile" "$nftNetifdPermFile"; do

I get some trailing whitespaces errors when I try to apply this on my local repo:

Thank you, these will be addressed in 1.2.1-r41

It looks like the latest changes break firewall.

root@GatewayDummi:~# service firewall restart
In file included from /dev/stdin:613:3-75:
/usr/share/nftables.d/chain-post/mangle_postrouting/30-pbr.nft:1:6-20: Error: Could not process rule: No such file or directory
jump pbr_postrouting comment "Jump into pbr postrouting chain";
     ^^^^^^^^^^^^^^^
/dev/stdin:4:12-14: Error: No such file or directory; did you mean table 'banIP' in family inet?
table inet fw4 {
           ^^^
The rendered ruleset contains errors, not doing firewall restart.

It looks like the latest changes break firewall.

root@GatewayDummi:~# service firewall restart
In file included from /dev/stdin:613:3-75:
/usr/share/nftables.d/chain-post/mangle_postrouting/30-pbr.nft:1:6-20: Error: Could not process rule: No such file or directory
jump pbr_postrouting comment "Jump into pbr postrouting chain";
     ^^^^^^^^^^^^^^^
/dev/stdin:4:12-14: Error: No such file or directory; did you mean table 'banIP' in family inet?
table inet fw4 {
           ^^^
The rendered ruleset contains errors, not doing firewall restart.

Not really a solution (yet) but
pbr_postrouting chain (and pbr_input) chain were removed on 30 September
stangri/pbr@481b71d

I checked and postrouting is also removed from LuCi so maybe you have old rules in your config using postrouting which are no longer valid?

Not really a solution (yet) but pbr_postrouting chain (and pbr_input) chain were removed on 30 September stangri/pbr@481b71d

But the file is still there in this repo:
https://github.com/openwrt/packages/blob/master/net/pbr/files/usr/share/nftables.d/chain-post/mangle_postrouting/30-pbr.nft

So maybe @stangri simply forgot to delete the files here?
Compared to his original repo, the packages repo here has some (orphaned?) files.