I revieweddraft-ietf-idr-entropy-label on behalf of the operations and managment area. 

   introduced by [RFC6790], and deprecated by [RFC7447].  The latter RFC
   specifies that the ELC attribute, BGP path attribute 28, "MUST be
   treated as any other unrecognized optional, transitive attribute".
   This specification revises that requirement.

it's worth noting that  intermediate nodes which apply RFC7447 behaviors or which are configured to strip attributes for self protection purposes (juniper has config knob for this) are broadly equivalent to stripping these attributes.

If the case described in 2.5 occurs

2.5.  Network Operation Considerations

   In the corner case where multiple nodes use the same IP address as
   their BGP next hop, such as with anycast nodes as described in
   [RFC4786], a BGP speaker MUST NOT advertise a given capability unless
   all nodes sharing this same IP address support this capability.

having some nodes lose or fail to pass attributes due to configuration is the equivalent of having the attributes not propagate to all nodes.

reading between the lines in 2.3

   By default, the RCA MUST NOT be accepted from peers not under the
   administrative control of the local network administrator (so,
   generally, from EBGP peers);

non-default enablement on ebgp peers is probably a relatively common case, as when you have a bunch of internally managed private asns part of a common administrative domain of control. so it is probably worth noting that case, e.g. mature adults will do this all the time.

if a border router doesn't
   implement this specification, the attribute, like all BGP optional
   transitive attributes, will propagate to neighboring Autonomous
   Systems.

This is an unfortunate form of information leakage. e.g. only supporting routers can be assumed follow instractions about not propagating said attributes. it hard of not impossible to comply with 2.3 instruction if you are unaware of teh attribute.

while I agree with  the discussion of 

ELCv3 Capability

that the value of exploiting this is dubious I would be concerned that adding new RCAs that perform other functions might be more adversely impacted.

thanks
joel