The $currentDate update operator is being deprecated for MongoDB 4.4. As an alternative users can rewrite their updates using updates with aggregation pipelines and can use the $$NOW and $$CLUSTER_TIME aggregation system variables to access the current wall-clock and cluster time.
In the following example, a document is updated using pipeline updates and $$NOW:
> db.test.insert({_id: 1, value: null, lastUpdated: null})
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WriteResult({ "nInserted" : 1 })
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> db.test.find()
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{ "_id" : 1, "value" : null, "lastUpdated" : null }
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> db.test.update({_id: 1}, [{$set: {value: 1, lastUpdated: "$$NOW"}}])
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WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })
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> db.test.find()
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{ "_id" : 1, "value" : 1, "lastUpdated" : ISODate("2019-10-01T13:27:55.583Z") }
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