June 04, 2019
Dear participants of the Monero ecosystem,
We would like to inform you that we will be phasing out long payment IDs this year. Long payment IDs are detrimental to privacy and a source of negative user experience (as well as additional support work for services). Services will have to upgrade to either integrated addresses or subaddresses. Note, however, that there is some discussion about phasing out integrated addresses as well. Therefore, services are, to avoid having to potentially perform additional work, encouraged to upgrade to subaddresses directly.
Long payment IDs will be phased out because they have several salient drawbacks. First, long payment IDs are detrimental to privacy insofar as they can potentially link the transactions of a user in case of reusage. Second, long payment IDs have to be attached separately to a transaction. This is inconsistent with conventional cryptocurrency transactions and therefore unintuitive for the user. As a result, users occasionally forget to attach the long payment ID when sending their transaction to a service and thus have to go through support to 'recover' their funds. Third, it logically follows from the previous point that long payment IDs cause additional support work for services. Fourth, only one long payment ID can be attached to a transaction. Thus, services cannot batch withdrawals of multiple users specifying a long payment ID. By contrast, subaddresses have no such restriction and withdrawals can thus all be batched.
In sum, payment IDs have serveral salient drawbacks and will therefore be phased out. Services are recommended to upgrade to subaddresses as soon as possible. Subaddresses essentially function similar to Bitcoin HD wallets and should thus be relatively straightforward to implement. Furthermore, subaddresses are managable from a resource point point of view.
The Monero dev community
An upgrade process PDF that may be useful for you can be found here: https://github.com/monero-project/meta/files/2775745/Upgrade.Process.pdf