We would like to design some complex forms which is not easy or feasible on SAIL

Hi Vikas,
Please go through the below links.
forum.appian.com/.../HTML_Forms,
forum.appian.com/.../PDF_Forms,
forum.appian.com/.../JSP_Forms

Note: Support for using an HTML, PDF, or JSP form is deprecated, not recommended to use.

It is very easy to get that done. Just some basic knowledge of HTML/PDF Form/JSP would be required. It is easy to transfer data between PVs and such custom forms. Usage of such forms is deprecated , but those custom forms execution may still be possible (I tried custom forms before they got deprecated).

Can Appian Engineering team confirm on what version they are planning to drop these options ?

Would you be able to provide an example of a feature for these forms that you consider not to be feasible in SAIL?

Vikas,
Can you share more details on the use case?

It's also possible to use Appian in headless mode. You can build your own UI with any technology you want. I did refactor an existing webapp to work with Appian instead of another BPM tool. You'll need to interact with Appian engine using Java API. A few drawbacks are: 1) Quite a bit of Java coding required. 2) Take care of your own support for mobile devices.

Just want to add to what others have already mentioned.
In the past, we created forms in the PDF using Adobe Acrobat, we developed a custom plugin to parse those PDF fields based on their ACRO field properties.

NB: You can read my SO question regarding how to read Acro field properties if you consider developing a custom PDF parser.
stackoverflow.com/.../how-to-get-acrofield-properties-using-itext

You have Adobe JavaScript also available if you want to add custom validations on the PDF form fields themselves.

Here is the link to Reference JavaScrip for Acrobat API www.adobe.com/.../js_api_reference.pdf

You can even invoke direct database calls from the PDF forms.

Thanks all for sharing your inputs, but is it worth to spend time on these options as they are deprecated and not sure how long will be in the product.

If you don't want to use SAIL forms, one thought - Create a PDF form and add Adobe javascript validations enabled on all the necessary fields in the PDF form. Upload this PDF form into Appian KC then on the user input form, give link to download this PDF form for your customers, which then they use to fill the data and upload into Appian from the task available to them. Once successful upload through the process, have a smart service that parses your PDF form and populates your process variables.
Cons: Maintainability - you need to have a customized plug-in to parse the PDF form.