2G and 3G have been sunset in few countries and other countries will be sunsetting them soon. VoLTE feature of LTE modules is necessary to be able to do calls over 4G without 2G/3G fallback need.
Native SMS can be transmitted or received over 2G/3G circuit switched or packet switched. when 2G/3G are completely removed and we are left with 4G and 5G only. is the only way for SMS to continue is the use of IMS or are there any different solutions?
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No. SMS over IMS is not the only way for the SMS to be transmitted in 4G/5G.
In both 4G and 5G (SA) architectures you can send an SMS in two ways: over IP/IMS, or over NAS (SG-SMS).
So for the equipment to send an SMS over IMS you need to first register on IMS (either VoLTE or VoWifi) including +g.3gpp.smsip parameter into the Contact header of your IMS SIP REGISTER.
If you do not inform SMS over IMS capability to the network in that registration, you'll send/receive SMS over NAS. It's very similar to an SMS sent over 2G/3G network, in the sense that the data/text is sent into a control channel and not a data channel.
The ideal way is to send SMS over IP/IMS (IMS is meant to transfer any type of data: text/audio/video/etc.), but there's the fallback solution in both 5GS and 4G networks which is SMS over NAS signaling. Because IMS capability varies a lot from operator to operator, the SMS over NAS is a very common scenario.
For more details, refer to the specifications: 3GPP TS 24.341 (Support of SMS over IP networks) and 3GPP TS 24.301 (Non-Access-Stratum protocol for EPS).
SMS can be sent over IMS using the IMS Core. If there is an existing LTE network but no IMS support, still the SMS can be sent or received over the SG interface in the LTE network.
Many operators use the SG interface to deliver and receive SMS.