Whydo legacy plans include a Planid but non-legacy plans do not? (4220513)

Only legacy plans include a Planid in TSQL. Non-legacy plans do not.

Example:
execute master.dbo.xp_slssqlmaint N'-PlanID F3D7F40C-AC85-4E94-9EF6-BC4A01CE3D88 -BkUpMedia DISK -BkUpDB -UseDefDir -BkExt "bak" -NOOPTOLR -CompressionLevel 2 -Logging 0 -Default "%D_%T_%Y-%m-%d-%H%M%S.%EXT%" -Exclude Offline LogShippng '

-vs.-

The databases to be backed up are listed individually or as "All user databases"
execute master.dbo.xp_slssqlmaint N'--D "Northwind" "AdventureWorks" "AdventureWorksDW" -BkUpMedia DISK -BkUpDB -UseDefDir -BkExt "bak" -NOOPTOLR -CompressionLevel 2 -Logging 0 -Default "%D_%T_%Y-%m-%d-%H%M%S.%EXT%" -Exclude Offline LogShippng