The APM server intake model does not include it: https://github.com/elastic/apm-server/blob/2c65c70cda74aa3eb4633c022417548ef52d89c7/model/modeldecoder/v2/model.go#L797-L846
It is only defined for spans: https://github.com/elastic/apm-server/blob/2c65c70cda74aa3eb4633c022417548ef52d89c7/model/modeldecoder/v2/model.go#L647-L648

The current Node.js agent is:

• sending it here:

  apm-agent-nodejs/lib/instrumentation/transaction.js

  Line 132 in 1207b49

  sync: this.sync,

• updating it here:

  apm-agent-nodejs/lib/instrumentation/index.js

  Line 363 in 1207b49

  if (trans) trans.sync = false

• and here:

  apm-agent-nodejs/lib/instrumentation/async-hooks.js

  Lines 97 to 99 in 1207b49

  	if (transaction && !transaction.ended) {
  	transaction.sync = false
  	}