It’s pretty much confirmed. The reason why Israel hasn’t retaliated to Iran yet and why the US is deploying THAAD is because Israel is critically low on interceptors. From the Financial Times:
“
Israel faces a looming shortage of interceptor missiles as it shores up air defences to protect the country from attacks by Iran and its proxies, according to industry executives, former military officials and analysts.
The US is racing to help close gaps in Israel's protective shield, announcing on Sunday the deployment of a Terminal High-Altitude Area Defense (Thaad) antimissile battery, ahead of an expected retaliatory strike from Israel on Iran that risks further regional escalation.
"Israel's munitions issue is serious," said Dana Stroul, a former senior US defence official with responsibility for the Middle East.”
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During the October 1 attack, there was a sense the IDF reserved some Arrow interceptors in case Iran fired its next salvo at Tel Aviv,” said Ehud Eilam, a former researcher at Israel’s Ministry of Defence. “It’s only a matter of time before Israel starts to run out of interceptors and has to prioritise how they are deployed.”