Soviet pilots in Egypt's War of Attrition (1969–1970)
Moscow sent some 15,000 personnel and full air-defense units to Egypt, including pilots who flew MiG-21s in combat against Israeli aircraft. The Soviet role was officially denied at the time. After a July 30, 1970 dogfight in which Israeli pilots downed five Soviet-flown MiGs, the operation was scaled back, but it took decades for Moscow to fully acknowledge it.
A ceasefire was signed in August 1970 that froze the Suez front. Israel quietly understood it had been fighting Soviet pilots.
The episode set a template for great-power 'deniable' combat involvement that allies later copied. Public admission came piecemeal, mostly after the Cold War.
Like Beijing's J-10CE engineers, Soviet personnel were on the ground (and in the cockpit) of a client state during a hot war while their government denied it. The pattern of years-later admission, once strategic value had been extracted, is the same.