My personal opinion that ISPs should ideally not remark DSCP. However, I do not feel strongly about this. If an ISP flattens all the DSCP to 0, it probably doesn't matter in practice. If I understood correctly, one major international network recently told me they flatten everything to DSCP 0.

I have implemented prioritization at layer 2. So we do ignore DSCP when determining the incoming traffic class of traffic. All Internet traffic gets CoS 0 (e.g. by "qos cos 0"). In my gear, I believe CoS 0 is actually traffic-class 1 under the hood, such that CoS 1 can be traffic-class 0 for below best effort. We do not use below best effort, though.

We have non-Internet traffic that gets prioritized. For example, SLAed point-to-point Ethernet circuits get a higher CoS value (again fixed, regardless of the customer's incoming p-bit or DSCP).

Prioritization is maintained through the network using VLAN p-bit values. That means I do MPLS-in-VLAN, which feels a bit odd and requires extra configuration (i.e. a separate "interface Vlan123"). I would like to use MPLS EXP bits, but my vendor does not seem to support explicit null labels. Without the VLAN p-bit, penultimate hop popping would thus lose the priority value to the last router. Priority still matters at the last router because there are switches and PON access gear behind the router.

So we should be transparent to DSCP. I think we have been when I've looked, but I can't remember for certain.

Again, take this all with a grain of salt. I'm just one guy at a little ISP. As always, if I'm doing something dumb, I welcome education from others.