I think he is talking about radio bearer only…
Do you see any case in 4G that RRC is released but bearer not released?
Radio bearer and EPS bearer are different things.
Radio bearer is always related with RRC connection.
Exactly 
.
Only erab will be released, i.e. drb + S5 bearer.
Erab + s5 bearer = eps bearer?
No sorry, I mean erab = drb+ S1 bearer.
Eps = erab+ S5/s8 bearer.
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Think it’s called hanging bearer or hanging session issue in Core networks.
Yes it is entirely possible and happens.
PGW are aware of bearer traffic and can (depending on vendor and feature) some time tear down bearer if no traffic is running through it.
From GSMA, I found 2 case “default IMS bearer is deleted”: UE roaming & IMS deregister.
Before default bearer is delete, RRC connection is release.
So my question is: in case inactive timer expire (10s), after VoLTE call ends 10s, whether enb send “RRC conn release” and default bearer is also deleted?
Small answer : No
Detailed answer: After VolTE call end, Dedicated IMS Bearer is released via NAS signalling, after 10 sec (or whatever Inactivity timer is), RRC Connection rel is received by UE & DRBs are released
DRB for IMS is released (all DRBs are released when RRC Connection release).
IMS PDN (Default Bearer) is still there till UE sends Detach/PDN Disconnect Request to NW.
Flow:
UE → BYE → NW
UE ← Dedicated Bearer Release ← NW
RRC Inactivity timer = x sec
After x sec,
UE ← RRC Conn Rel ← NW
DRBs (all default bearers) Released
UE still has IMS & Default Internet PDN active, for which it can receive PS paging, UE can also send Service request to access Data or make IMS Call
For NAS IMS Default Bearer release, the following is procedure:
UE → PDN Disconnect Request (EBI) ->NW
UE ← Deactivate EPS Bearer Ctxt Req ← NW
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So bearer is just a virtual concept?
RAB is the link between Core and UE.
Perfect. 
Nicely put together! 
And that is why the famous sentence about LTE: “LTE provides an always ON connectivity”.
Exactly…
Agree with @RMatolia
Adding one more point, along with DRB1 (of QCI9) & DRB2 (of QCI5) corresponding S1 bearers will also be released once UE Inactivity is detected by eNB.